Optical waveguides are incorporated in optical integrated circuits for a variety of purposes. In particular, optical waveguides are utilized to form resonator based devices such as receivers, lasers, transmitters, modulators and interferometers. Different types of resonators may be incorporated in these devices such as a linear resonator which comprises a linear waveguide and a reflecting mirror placed on each end of the linear waveguide or a ring resonator which includes a waveguide having the geometry of a ring.
An optical resonator resonates when the round trip phase shift of optical radiation of a particular wavelength propagating in the resonator waveguide equals an integer multiple of 2.pi.. The round trip phase shift in a resonator may be represented by .omega.p/c, where .omega. is the angular frequency of the radiation and p is the round trip optical path-length of the resonator. Thus, the resonant wavelength of a particular optical resonator is strongly dependent on its optical path length.
In many applications, it is desirable to tune a resonator to resonate at a particular wavelength. For example, it might be desirable to tune a laser to generate optical radiation at a particular wavelength. In addition, in a multiple wavelength optical communication system (e.g., as described in the prior art, U.S. Pat. No. 4,592,043, "Wavelength Division Multiplexing Optical Communications", by Gareth F. Williams), it might be desirable to tune a resonator to a specific wavelength to select the specific wavelength out of a plurality of wavelengths arriving at the receiver.
Other applications use waveguide interferometers as intensity modulators. For example, a typical Mach-Zehnder waveguide modulator comprises two waveguides coupled to each other at two points by two evanescent couplers. The modulator has a transmission maximum when the paths in the two waveguides differ by an integral number of optical wavelength and a transmission minimum if the paths in the two waveguides differ by a half-integral number of optical wavelengths. Thus, it is desirable to control the transmission of the modulator by varying the difference between the optical path lengths in the two waveguides.
In other applications, such as coherent lightwave receivers, it is desirable to vary the phase of an optical signal or radiation. This may be done by varying an optical path length and thereby the phase.
In conventional optical integrated circuits, waveguides may be manufactured with lithium niobate materials. At present, waveguides formed from these materials have unacceptably high losses for many applications. In optical integrated circuits, waveguides may also be formed from glass materials. However, there is presently no suitable way to alter the optical path length of a glass waveguide formed in an optical integrated circuit device, for example, to form a tunable resonator. Accordingly, it is an object of the present invention to provide an optical integrated circuit including a low-loss waveguide and a means for varying the optical path length of the waveguide.
In the prior art (see, e.g., D. L. Franzen and E. M. Kim, "Long Optical-fiber Fabry-Perot Interferometers," Applied Optics, vol. 20, No. 23, pp. 3991-3992, 1981; S. J. Petuchowski, T. G. Giallorenzi, and S. K. Sheem, "A Sensitive Fiber-Optic Fabry-Perot Interferometer," IEEE Journal of Quantum Electronics, vol. QE-17, no. 11, pp. 2168-2170, 1981; and J. Stone, "Optical-Fiber Fabry-Perot Interferometer With Finesse of 300," Electronics Letters, vol. 21, pp. 50-4505, 1985), a piezo-electric device has been coupled to an optical fiber to modulate the length of the optical fiber. However, a piezo-electric material has not heretofore been incorporated in an optical integrated circuit to alter the optical path length of a waveguide.
Accordingly, it is a further object of the present invention to provide an optical integrated circuit incorporating a piezo-electric (PZT) material for changing the optical path length of a waveguide.